There are numerous types of contaminants found in water, including dissolved ionic species. Deionization, also called demineralization or ion-exchange, is the removal of ions and minerals from an aqueous source by means of exchange media which have an affinity for dissolved ionized salts. These media are divided into two classifications, namely cationic media which remove the positive ions and anionic media which remove negative ions. In such media, pendant ionic groups are presented in such a way that: the pendant ions are easily replaced by the ionic species in the aqueous source.
In recent years, the technology of water demineralization has been advanced through use of reverse osmosis membranes, that can provide a medium for separation. Reverse osmosis uses hydraulic pressure to overcome the normal osmotic gradient thus a physiochemical interaction between salt and the membrane cause salts to be rejected and flow to the more concentrated compartment; only water passes through the membrane and into the less concentrated or clean compartment. Although the designs provide for the movement of water through the membrane for flushing of the surface, colloidal species still can provide a fouling problem
Electro-dialysis membranes, which also have been recently introduced, provide yet another means for separation of impurities from water. Such membranes rely on electricity to force ions across the membrane, thereby achieving a removal of ionic species from water.
More recently, it has been disclosed that ionic separation procedures may be carried out using a continuous deionized ion exchange bead media in a compartment which is in electrical contact with an ion permeable membrane such as shown in U.S. Pat. No. 5,004,543.
Another example of separation process is provided in U.S. Pat. No. 5,078,842, which discloses separation in an electrochemical cell which contains an ion exchange resin compartment enclosed by a cation permeable membrane. A current is applied externally to the resin chamber by an anode in an anode compartment and a cathode in a cathode compartment. The opposite electrodes to drive the ionically bound ions through the cation permeable membrane and toward the cathode compartment.
All of the foregoing separations suffer from a major limitation, namely, membrane fouling. More specifically, the fouling of water cleaning membranes by colloidal particles is a significant factor that limits operating efficiency in sea water or brackish water desalting, as well as in industrial waste water cleanup. For example, during the operation of an electro-dialysis membrane, the operational efficiency declines as a result of the formation of cake-like layer on the "dirty water" side of the membrane. Similarly, the capacity of ionic separation using the continuous deionized ion exchange bead media is generally limited due to colloidal fouling. Thus, frequent replacement and/or regeneration of the ion permeable membranes is required when any substantial quantity of contaminated aqueous streams are to be cleaned.
Accordingly, there remains a need for new processes and apparatuses which may be used to effect separation of ionic species from aqueous streams.